The present invention relates to radar systems and, more particularly, to laser radar systems.
The following references are referred to hereinafter, and the teachings thereof as employed within the present invention are incorporated herein by reference to avoid redundancy and undue complication of this application:
[1] Claude R. Cooke, "Laser Radar Systems, Some Examples", SPIE Vol. 128, pp 103-107 (1977).
[2] P. A. Forrester, K. F. Hulme, Review Laser Rangefinders, Optical and Quantum Electronics, Vol. B, pp 259-293 (1981).
[3] P. M. Woodward, "Probability and Information Theory, with Applications to Radar", Pergamon Press, Oxford, 1953.
[4] C. E. Cook and M. Bernfeld, "Radar Signals an Introduction to Theory and Application", Academic Press, (1967).
[5] K. F. Hulme, B. S. Collins, G. D. Constant, J. T. Pinson, "A CO.sub.2 Laser Rangefinder Using Heterodyne Detection and Chirp Pulse Compression", Optical and Quantum Electronics, Vol. 13, pp 35-45 (1981).
[6] J. E. Kiefer, T. A. Nussmeier, and F. E. Goodwin, "Intracavity CdTe Modulators for CO.sub.2 Lasers", IEEE Journal of Quantum Electronics, Vol. QE-8, No. 2, pp 171-179, February 1972.
[7] A. Yariv, "Quantum Electronics", John Wiley & Sons Inc., Second Edition, 1975.
[8] A. VanLerberghe, S. Avrillier, and C. J. Borde, "High Stability CW Waveguide CO.sub.2 Laser for High Resolution Saturation Spectroscopy", IEEE, Journal of Quantum Electronics, Vol. QE-14, No. 7 pp 481-486, July 1978.
[9] J. R. Klauder, A. C. Price, S. Darlington, and W. J. Albersheim, "The Theory and Design of Chirp Radars", The Bell System Technical Journal, Vol. 34, No. 4, pp 745-808, July 1960.
Radar systems have been employed in both non-military and military applications for a good number of years. In recent years, the technical advances required in radar systems for the military have been considerable. When first employed in World War II, the accuracy and speed of radar were not overly critical. The radar was used to detect slow-moving ships and planes which could then be engaged on a not-too-critical time basis. More recently, speed and accuracy have become critical in order to allow the timely detection and destruction of smaller, fast-moving missiles having high destructive power. In addition, these contemporary radar systems are required to perform in an environment filled with highly efficient countermeasure devices.
Because of its resistance to jamming and interference from outside sources and its superior range and angular accuracy, light has replaced electro-magnetic energy in many applications from the telephone on up. The application of lasers to the radar function is, therefore, logical.
Laser rangefinder techniques have been successfully shown to determine ranges of targets at distances up to five miles with accuracy of 2 to 10 meters [1]. As with conventional radar systems, laser systems may be classified into two basic categories: (1) Direct, or incoherent detection, and (2) Heterodyne or coherent detection.
The theory of the former dictates that the best signal-to-noise ratio is provided when the transmitted energy is concentrated into the shortest possible pulse [2]. This yields a good range measuring and resolution capability. On the other hand, coherent detection requires highest possible average transmitted power for best signal-to-noise, irrespective of pulsewidth. In addition, accurate target radial velocity measurements can be obtained in the latter case. Woodward [3] pointed out, during the early stages of modern radar technology, that radar resolution and accuracy were functions of the signal bandwidth, being AM or FM in nature, regardless of the transmitter waveform. Thus a continuous power, or long pulse mode of operation heterodyne system may also yield good range measuring and resolution capability when using a wideband signal. A more complex receiver is required to extract the wide band information in this type of signal, as opposed to the direct detection system. These receivers, as used in conventional radar systems, are designated as matched-filter signal processing. The advantages of such signal processing techniques in radars, as pointed out by Cook [4], are:
1. More efficient use of the average power available at the transmitter.
2. Increased system accuracy, both in ranging and velocity measurements.
3. Reduction of jamming vulnerability.
In light of the above, it would seem that heterodyne detection scheme, using wideband signal, would also be beneficial to laser radar systems.
In a recent pulication [5], a laser range using heterodyne detection and chirp pulse compression is described. The wideband signal consists of a linear FM chirp pulse of relative long duration. The matched-filter, at the receiver end is a Surface Acoustic Wave (SAW) device, which compresses the relative long FM chirp pulse into a narrow one (of the same bandwidth), from which the range and velocity information may be extracted. The duration of the compressed pulse is approximately the inverse of the bandwidth of the original signal. Thus, as the amount of frequency that is chirped increases, so does the resolution of the range and velocity measurements. In the above work, as described in greater detail hereinafter, an acousto-optic modulator, placed at the output of the laser, was used to obtain a chirp width of 14 MHz. This would yield a compressed pulse of the order of 100 ns. Further increases in frequency deviation, with attendant reduction in pulse width, are limited by transit time of the acoustic wave across the optical beam.
Huhne et al (Optical and Quantum Electronics 13, 1981, 35-45), for example, have demonstrated a CO.sub.2 laser range finder using FM chirp modulation and pulse compression. They modulate the laser outside the laser cavity using acoustic optic (AO) modulation. It is an object of the present invention to provide an order of magnitude increase in range resolution because of wider frequency deviation employing an electro-optic (EO) modulator and a high pressure CO.sub.2 laser.
The Huhne et al. approach is shown in simplified form in FIG. 1. The laser is shown generally as 10 and comprises, in the usual manner, a high reflectivity end mirror 19 and an output mirror 20 on the other end between which the laser gain medium generally indicated as 22 is contained. As the laser light 18 emerges from the cavity of the laser 10 through mirror 20, it passes through the acousto-optic element 24 which is driven by modulating driver 26, which imparts the modulation thereto.
Stein (IEE J. of Quantum Electronics, Aug. 1975, 630-31) previously used intracavity EO modulation to chirp a high pressure laser in a radar system that does not use pulse compression. However, he only achieved a 20 MHZ chirp with a linear ramp. He makes no mention of the specific percent linearity achieved. From the graphical data he provides, however, it is apparent that an attempt at pulse compression of the type described herein would not have been successful. This difficulty with precise linearity was also encountered by Hulme and Collins [Society of Photo-Optical Instrumentation Engineers' Proceedings; Vol. 236, pp. 135-138]. It is, in fact, this finding that compelled them to use acousto-optic modulation as described above. It is another object of the present invention to provide high linearity of about 100 MHz thus making usable pulse compression possible.
The Stein apparatus is shown in simplified form in FIG. 2 and is similar to the Huhne et al. apparatus, with the exception that the acousto-optic element 24 thereof is replaced by an electro-optic crystal 28 of a conventional variety, which is placed within the cavity of the laser 10 ahead of the output mirror 20.
Wherefore, it is the object of the present invention to provide a laser radar which overcomes the limitations of the prior art described above.